Anechoic chambers typically have their interior surfaces covered with wedge-shaped and pyramid-shaped blocks of polyurethane foam injected with carbon. Carbon acts as a lossy material absorbing electromagnetic radiation (EMR). These foam blocks are typically 6 feet (1.8 m) tall when testing in certain frequency ranges, such as 200 MHz to 500 MHz and up. The wedge-shaped blocks are used along the side walls, ceiling and floor while pyramid-shaped blocks are used at either end of the room, that is, behind the radiation source and behind the target. Pyramid-shaped blocks are used whenever radiation is expected to strike the surface generally straight on. Therefore the pyramid-shaped blocks are also used along the side walls near each end of the chamber.
Conventional foam absorbers, in particular the pyramid-shaped absorbers, are quite heavy, can sag (reducing their effectiveness and requiring replacement) and take up a substantial amount of space. The large size of the foam wedges and pyramids forces the use of a great deal of absorber material and a large room to create only a modest size chamber. The weight of the foam absorbers mandates that the walls and ceiling be made quite strong, thus increasing the cost of construction.